A wide variety of osseous lesions are amendable by radical en bloc resection and prosthetic bone and joint replacement using custom-designed implants. The objectives of this project are: 1) to establish design guidelines for such implants, 2) to apply bone in-growth material to the prosthesis for prolonged fixation without cement application, 3) to quantitate joint functional changes due to prosthetic replacement and soft tissue reconstruction, and 4) to develop better rehabilitation programs for functional improvement and implant protection. There are 131 cases involving the use of the custom prostheses in the areas of the hip, knee, shoulder, and segmental long bone to date. Seventy percent of these patients had either primary or metastatic bone tumors with the longest follow-up of 112 months. Twenty-seven patients are deceased, due primarily to metastatic disease, but without local recurrence at the replacement sites. A complete prosthesis system using cement fixation for each anatomical region involved has been developed. The segmental bone prosthesis using bone in-growth material has been tried on six patients with extremely promising results. The functional results of these patients were near normal. Complications occurred in a few cases with stem fracture, bone-prosthesis interface loosening, infection, bone fracture, and dislocation. All of these cases required revision with subsequent good results. The major reasons for failure were attributed to poor prosthetic design, lack of optimal fitting and fatigue failure. Improved implant, better prosthetic material selection and refined surgical techniques and rehabilitation are being developed through the present research program to obtain better results.